[Xpert]But *why* no vblank?

[Scott Long]

I must have seen about 10 requests/complaints on this list about no vblank sync support since I subscribed. The reply is always the same, that the vblank signal comes on an interrupt, and a usermode program such as X cannot get anywhere near it.

I assume there are a number of issues surrounding vblank support. First, how does vblank fit into a network-transparent graphics protocol like X? How would the client request a particular operation to be sync'd to vblank? Obviously this syncing should occur on the server side, since a remote client can't receive the vblank signal for the local display.

Also, how could the actual sync be performed? Could we have a device like /dev/vblank that blocks on read until the vblank interrupt is sent? Or some kind of ioctl?

And what are the ramifications of having video code running in both the kernel and the X server? Will kernel-level video drivers interfere with X's drivers? How does X interact with the fbdev, currently?

I realize that I've just asked a bunch of questions without really answering anything. But it's very frustrating to keep reading these requests for vertical sync support, and keep seeing answers that say "That's just impossible, sorry."

Oh, come on. We all know it isn't impossible; it's just a hairy problem that no one seems to want to deal with. It involves a bunch of compromises that people don't want to make. But this is an extremely *basic* feature that people are demanding and it doesn't seem right to just blow them off.

Scott Long
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